During a printing cycle in a conventional screen printing operation, it is necessary normally to move a pool of printing ink across a screen toward one end of it, in a "flood stroke", then to position the screen immediately on top of the work, lift the flood bar and lower a squeegee bar extending across the width of the screen into contact with the screen. Then, the rubber squeegee is drawn in the opposite direction across the screen to force ink through the pores of the screen in the areas desired so as to print the underlying work corresponding to those image areas of the screen through which ink has been forced.
Usually, the flood bar and squeegee are mounted on a carriage which is reciprocable along a pivotally mounted printing head. Printing with the squeegee is effected as the carriage travels from the front to the rear of the printing head. Upon reaching the rear end of the printing head, the carriage reverses direction in order to flood the screen and carry the squeegee back to its starting position at the front end of the printing head. The path of the carriage from the front end to the rear end of the printing head is defined as the printing stroke. In order to produce prints of varying size and length and of multiple colors, it is desirable to vary the length of the printing stroke as well as the position of the front and rear ends or stroke positions respectively of the printing stroke.
Various types of apparatus have been developed to reciprocate the carriage carrying the flood bar and the squeegee across the screen and to reverse their positions during the press cycle. Such apparatus generally includes an electric motor which drives the carriage through some type of mechanical linkage therebetween.
Various types of systems have been used for adjusting the length of the printing stroke and the front and rear ends thereof. Electrical sensors have been used to manipulate operation of the motor which drives the carriage. Adjustment of these systems, however, proved to be time consuming for the operator who had to make the adjustments individually and manually at locations on the carriage or printing head, as the case might be.
A reliable system was found to be one in which the length of the printing stroke is varied by adjusting the mechanical linkage between the drive motor and the carriage. One mechanism of this type is described in U.S. Pat. No. 3,859,917 to Bubley, et al. Although reliable, adjustment of the printing stroke required stopping the press and removing access covers to the linkage inside the press. This manipulation of the linkage was disadvantageous and time consuming.
Other means resorted to for controlling the carriage so as to adjust the length of the printing stroke comprised movable sensing devices positioned along the length of the printing head. These took the form of photo-sensors, photo-switches, proximity switches, and limit switches to set the limits of the stroke by manually positioning the front and rear stop positions of the carriage. Such sensing devices, however, required mechanical changing of their positions each time adjustment in stroke length was desired. After the mechanical adjustment of the sensor's position was made, the results or accuracy of the adjustment could not be ascertained except after a trial run. This was time consuming and subject to adjustment error because of the manipulation and manual operation required to achieve the desired adjustments.
It therefore would be desirable to provide a sensor system which readily permits selective adjustment of the length of the printing stroke as well as the relative positions of the front and rear ends or stroke positions of the printing stroke. It would also be desirable to be able to adjust the printing stroke without the necessity of manual manipulation of the reference sensors or of the carriage drive linkages. It further would be desirable that the adjustment of the printing stroke be readily accomplished by positioning the carriage in the front and rear stroke positions proximate the image areas of the screen through which the ink will pass. In addition, it is desirable that these adjustments be accomplished rapidly and economically.